Field of the Invention
The invention relates to a semiconductor device and method for fabricating the same, and in particular to a semiconductor device with a metal-oxide-semiconductor field effect transistor and method for fabricating the same.
Description of the Related Art
In the semiconductor process of a metal-oxide-semiconductor field effect transistor (MOSFET), there are electrons in an electric field because they are attracted by the positive electric potential, and the electrons will gain kinetic energy and become accelerated. For example, when the channel length of the MOSFET decreases, if the applied voltage stays unchanged, the lateral electric field in the channel will increase. As a result, after being accelerated by the lateral electric field, the energy of the electrons will increase greatly. Because the energy of the electrons accelerated by the lateral electric field is greater than that of the electrons in a state of thermal equilibrium, the electrons with more energy may be regarded as hot carriers. Hot carriers may cause impact ionization, particularly in the region where the channel connects to the drain region. When the energy of the hot carriers is greater than the energy barrier between the channel and the gate dielectric layer (Si—SiO2), the ionized electrons/holes impacted by the hot carriers may be injected into the gate electrode and cause a hot carrier injection (HCI) effect. While experiencing the HCI effect, the electrical properties of the device may be unstable, leading to the electrical properties of the device having a decreased reliability level.
In general, with conventional processing, the electric field of the region where the channel connects to the drain region is decreased by a lightly-doped drain being disposed so as to reduce the HCI effect. However, the HCI effect becomes worse as the driving current gets larger due to scaling-down of the size of transistor devices, and so a new method of processing is needed to reduce the HCI effect.